The bacteriostat 2,4,4'-trichloro-2'-hydroxydiphenylether, covered by U.S. Pat. No. 3,506,720, is annually marketed in million-pound quantities under the trademark IRGASAN DP-300.RTM. for use in soaps and cosmetics. It has been produced from 2,4,4'-trichloro-2-aminodiphenylether (TADE) by diazotization and hydrolysis according to Equation 1: ##STR1##
Products of this synthesis, in addition to the desired phenol (DP-300), include 24-28% 2,4,8-trichlorodibenzofuran (TCDBF) and traces of more than ten other compounds including phenolics and non-phenolics related to or derived from the furan compound.
Because of strict cosmetic industry and FDA specifications, the products of the above synthesis have required extensive and intensive purification procedures including high-vacuum distillation, precipitation, filtration and solvent recrystallization leaving a yield (based on TADE) of about 53.5% IRGASAN DP-300.
The major compounds found as impurities in the synthesis according to Equation 1 and their concentration limits in the commercial product are as follows:
______________________________________ 2,4-Dichlorophenol (2,4-DCP) &lt;0.01% 4-Chlorophenol (4-CP) &lt;0.01% 4,4-Dichloro-2-hydroxydiphenylether &lt;0.7% (DCHDPE) 2,4,8-Trichlorodibenzofuran &lt;10 ppm (TCDBF) 2,8-Dichlorodibenzofuran &lt;1 ppm (DCDBF) ______________________________________
In the previous procedures, these standards of purity were achieved by diazotizing molten 2,4,4'-trichloro-2'-aminodiphenylether with nitrosyl sulfuric acid/sulfuric acid at 55.degree.-60.degree. C. The diazotization reaction mass was hydrolyzed by 75% sulfuric acid at 155.degree.-160.degree. C. The organic phase was separated from the acid at about 120.degree. C. The organic (upper) phase was adjusted to pH &gt;12 with 50% caustic to form a slurry containing the water-soluble sodium salt of IRGASAN DP-300 and precipitated furan (TCDBF). The slurry was filtered to remove most of the furan by-product. The alkaline filtrate was extracted with toluene to remove most of the associated non-phenolic impurities and then the product was acidified to form an oily lower organic phase containing the crude product and an aqueous phase containing some of the phenolic impurities. The organic phase was distilled, separating the fore run containing phenolic impurities up to 180.degree. C./1 mm from the main cut at 180.degree.-225.degree. C./1 mm. The main cut of distilled IRGASAN DP-300 was recrystallized from an aliphatic petroleum solvent (NAPHTHOLITE 66/3) boiling in the range 130.degree.-138.degree. C. to yield a product meeting the above noted purity specifications. It is a slightly yellow crystalline material with a phenolic odor.